1 Answer
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It's not past tense (ta-form), but it's tari-form used to list two or more verbs and i-adjectives. So 「何も無かったり、有ったり」 roughly means "sometimes there is nothing, sometimes there is something" or "there may or may not be something".

Another question: is the speaker doubting whether 過去の遺物の集積場 exists? As in, does the sentence mean, there might/might not be a place where relics of the past accumulate? EDIT: My bad, they're doubting what's in the forest, like you said
– user30175Apr 3 at 17:56

@kumichan The subject of ある/ない is 何 ("something"), not 集積場.
– narutoApr 4 at 6:53